1. Field of the Invention
The present invention relates to an exhaust gas recirculation (EGR) system for a turbocharged, internal combustion engine.
2. Description of the Related Art
Exhaust gas recirculation is well known for internal combustion engines where part of the exhaust gas discharged from an engine is recirculated to the intake passage and injected back into the combustion chambers, along with air and fuel, to decrease the combustion temperature thereby reducing the amount of nitrogen oxides in the exhaust gas.
Two principle means of implementation of exhaust gas recirculation have been suggested for application to turbocharged diesel engines. In the first method, known as xe2x80x9clow-pressurexe2x80x9d loop, exhaust gas is routed from the turbine outlet to the compressor inlet. This method suffers from the drawbacks of fouling the compressor wheel and housing with exhaust deposits, possible overheating of the compressor wheel and the potential for severe fouling of the air-to-air charge air cooler. In the second method, known as xe2x80x9chigh-pressurexe2x80x9d loop, exhaust gas is routed from the exhaust manifold, before the turbocharger turbine, directly into the engine""s intake manifold (thereby eliminating the fouling potential of the low-pressure loop). This method suffers from the drawback that the pressure in the exhaust manifold must be greater than the pressure in the intake manifold. Most well-developed, heavy-duty, turbocharged diesel engines operate with intake manifold pressures that are higher than the exhaust manifold pressures. This partially accounts for the diesel engines"" excellent fuel economy characteristics. To cause the engine""s exhaust pressure to be higher than the intake pressure requires that a relatively inefficient turbocharger configuration be fitted to the engine or a back pressure device be fitted following the turbocharger""s turbine stage to cause the exhaust pressure to be higher than the intake manifold pressure. Poor fuel economy will be expected for either of these types of high-pressure loop arrangements.
The EGR system of the present invention uses a high-pressure loop in a manner that enables the intake manifold pressure to remain higher than the exhaust manifold pressure. This is accomplished by adding a separate EGR manifold and an additional exhaust valve for each combustion chamber, referred to herein as a xe2x80x98secondary exhaust valvexe2x80x99, that permits passage of exhaust gas from the combustion chamber to the EGR manifold. The secondary exhaust valve can be actuated by a mechanical, hydromechanical or electro-hydromechanical actuator in such a way that the valve can be opened and closed as a function of the rotational position of the engine""s crankshaft. The opening of the secondary exhaust valve occurs during the expansion stroke of the engine cycle, after the combustion process has been completed. The valve is closed at a point near the opening of the primary exhaust valve or valves.
Exhaust gas exits from the EGR manifold to the intake manifold, or other conduit for pressurized air, downstream of the turbocharger compressor. Exhaust gas exit is provided by an EGR valve that is controlled so that a pressure can be created in the EGR manifold and controlled to a higher level than exists at any moment in the intake manifold. The EGR valve can be controlled so that the amount of exhaust gas being fed into the intake manifold can be controlled over a wide range of flow rates as desired for optimum levels of emissions reduction and minimal fuel consumption penalty. The EGR valve is controlled by the engine""s ECU (engine control unit).
A decompression brake valve can be provided with the EGR system that connects the EGR manifold and the engine""s exhaust manifold. When the decompression brake valve is held open and the EGR valve is closed, the secondary exhaust valve in each combustion chamber can be opened at the beginning of the expansion stroke and closed at the end of the expansion stroke such that engine decompression braking is achieved.
With the EGR system of the present invention, the engine""s exhaust manifold and turbocharger can operate in the same type of efficient manner as is characteristic of current turbocharged engines without an EGR system. It is believed that with such a system, the engine will operate with better fuel economy as compared to engines equipped with any of the EGR systems known in the art at this time.
A further feature of the invention provides two EGR valves at the outlet of the EGR manifold. One valve allows the exhaust gas to be directed into the intake manifold without passing through a cooler. This will enable the diesel engine to start at lower ambient temperatures and warm up more quickly than engines without such a controllable exhaust gas recirculation system.